©• 



43 



When this capping surmounts a portion Avhich is more argilla- 

 ceous, the siliceous hardening and glazing gives it the appearance 

 of baked earthenware. Veins of chalcedony are common among- 

 the planes of shrinkage, and nests of the same material furnish 

 the so-called flint fragments which are so common throughout the 

 region. This chalcedony varies from pure translucent specimens, 

 to the porcelainised sandstone just referred to, and it is easy to 

 select pieces showing the different stages of transition. 



This gritty bed represents the surface of the country before it 

 w^as exposed to the erosive action of running water, and the 

 height of the flat-topped hills attest the depth to Avliich the creeks, 

 and wind have changed tlie contour of the country. From 

 Charlotte Waters to Lake Eyre the erosion has extended down 

 through the grits and for some distance into the blue clay, bein 

 deepest in the latter bed as Lake Eyre is approached. As a con 

 .sequence the nearer the lake the greater is the exposed or un- 

 covered surface of the clay through removal of the grits; the 

 only vestiges of which now are the porcelain stones and iron- 

 stone nodules which constitute the " gibbers " or stony patches 

 referred to previously as forming a feature of the surface of the 

 great plain. Along the railway line at present constructing the 

 only stone available for ballast are the gibbers, and but for their 

 existence ballast would require to be brought from places beyond 

 the plain. As it is they are cheaply and expeditiously raked to- 

 gether and wheeled on to the embankment. 



This continuous bed of clay, whose surface is below sea level at 

 Lake Eyre, abuts directly against the ancient rocks which form 

 the Flinders Kange and their extension north-westerly to the 

 Peake and Mount Dutton, which thus forms the southern edge 

 of the basin which encloses them. Northwards the clay-bed 

 extends into regions where the rainfall is considerable and the 

 ground more elevated, but over wdiich water seldom runs at the 

 surface. At whatever point, therefore, the lower side of this bed 

 is l^rought near the surface of the ground, opportunity is afforded 

 for waters to percolate below. This water having percolated to 

 the limits of the basin, where it is bounded by imper^ ious beds 

 of older rock but little jointed, must necessarily remain pent 

 under considerable hydrostatic pressure, which at length over- 

 coming resistance, forces a way up through the clay in the neigh- 

 l^ourhood of its abutment against the old walls. Lime is deposited 

 around the mouth of the orifice as soon as the water comes to the 

 surface, and the pressure eased. In this manner a mound is built 

 up of travertine, which inci'eases in dimension until the hydro- 

 static pressure is compensated, or, what is still more probable, 

 until the orifice is closed by continual deposition along its sides,, 

 and then another spring l^reaks out at a lower level. 



